Walnut Husk Rotting with Worms

Q. Our walnut tree is at least 10 years old and producing wonderfully for the past 3 years. This year toward the end of the season, the husks started getting black. I opened one and saw little white worms. I opened the nut shell and saw no damage to the nut itself, so we did nothing. By the end of the season, most of the husks were on the ground, all black. Of the husks with nuts, the nuts were okay but there were a lot of husks with dried up black nuts. We also noticed this larger branch on the tree that seems to have split its bark.

Damage to husk of English walnut

Damage to English a walnut limb

A. This sounds like walnut husk fly damage to the husk. If walnut husk fly damage starts early enough to the husk they can cause a nut failure just like you describe. If damage to the husk is later in the summer, the nuts inside will fully develop.

Walnut husk fly adult is around the size of a housefly and lays eggs just below the surface of the husk. This blackens the outside of the green, immature husk and causes it to get soft. Eggs from walnut husk fly hatch into maggots, or worms as you call them, feeding on the inside of the husk.

After a few weeks, older maggots fall from the husk to the ground and burrow into the soil where they spend the rest of their life before emerging as adults the following summer. They emerge as very colorful, mature flies ready to repeat the cycle. This lifecycle is repeated once a year with egg laying on husk starting in mid to late summer.

            The University of California has an excellent fact sheet on the Internet concerning the walnut husk fly and located at http://www.ipm.ucdavis.edu/PMG/r881301211.html. Control measures are mentioned but a bit difficult. If there are other walnut trees in the area this could cause an increase in the population of this pest in coming years if they are left uncontrolled.

Damage to tree limb

The pictures of the tree limb you sent look like borer damage to me. The same borers that attack fruit trees such as peach also attack walnut. Pull off the loose bark from the limb and look for damage from borers on the surface of the exposed wood.

If borers damage is present, remove all of the loose bark with a sharp knife. If the damage to the limb is more than halfway around its circumference, remove the limb. If it is less than half, the tree has a good chance of recovery if all the loose bark is removed down to healthy wood.